Iridium plating

ABSTRACT

An iridium electroplating bath is prepared by digesting an aqueous solution of iridium trichloride and sulfamic acid in a molar ratio of sulfamic acid to iridium of at least 7 at 100* C. for an extended period of time then adjusting the iridium content of the bath to about 3 to 20 grams per liter, adding about 3 to 20 grams per liter of ammonium sulfamate and electrodepositing iridium from the bath upon a conductive substrate with a bath pH of about 1 to 7 (adjusted by ammonia additions) and at a temperature of about 50* to 75* C. with insoluble anodes and a cathode current density of about 0.1 to 8 amperes per square decimeter to produce adherent crack-free iridium deposits.

United States Patent Skomoroski [54] IRIDIUM PLATING 72 Inventor: RobertM. SkomoroskLPaterson, NJ.

[73] Assignee: The International Nickel Company, Inc.,

New York, N.Y.

[22] Filed: Dec. 24, 1969 [2]] Appl. No.: 888,039

Gordon A. Conn, Plating, pp. 1258- l26l, Dec. 1965.

[ 1 Feb. 1,1972

Primary ExaminerG. L. Kaplan Attorney-Maurice L. Pinel [5 7] ABSTRACT Aniridium electroplating bath is prepared by digesting an aqueous solutionof iridium trichloride and sulfamic acid in a molar ratio of sulfamicacid to iridium of at least 7 at [00 C. for an extended period of timethen adjusting the iridium content of the bath to about 3 to 20 gramsper liter, adding about 3 to 20 grams per liter of ammonium suliamateand electrodepositing iridium from the bath upon a conductive substratewith a bath pH of about I to 7 (adjusted by ammonia additions) and at atemperature of about 50 to 75 C. with insoluble anodes and a cathodecurrent density of about 0.1 to 8 amperes per square decimeter toproduce adherent crack-free iridium deposits. 1

4 Claims, No Drawings llRIDlUll/l PLATING The present invention isdirected to the electrodeposition of iridium. lridium is a platinumgroup metal which is hard and dense and has a highmelting point,together with excellent resistance to high temperature oxidation. It hasbeen found that iridium provides a desirable catalytic effect ofparticular interest in antismog devices for use in conjunction withinternal combustion engine exhaust gases. Insofar as theelectrodeposition of iridium is concerned, the art indicates that littleprior work has been done. Aqueous electroplating baths are available forthe electrodeposition of iridium but the available baths are stillsubject to difficulty. Thus, existing baths are subject to instabilityat some operating conditions and in many cases it has been found notpossible to produce adherent iridium deposits on substrates such asstainless steel using such baths.

It is an object of the present invention to provide an iridiumelectroplating bath which is stable and which yields iridium depositswhich adhere firmly to metal substrates of various types, includingstainless steel.

in accordance with the invention, an iridium electroplating bath isprepared by digesting an aqueous solution of iridium trichloride withsulfamic acid in a molar ratio of sulfamic acid to iridium of about 7 toabout 44 for at least about hours at a temperature of about 100 C. Thesolution containing the digestion product of iridium trichloride andsulfamic acid is then adjusted to provide an iridium content of about 3to about grams per liter. Ammonium sulfamate in the amount of about 3 toabout 20 grams per liter is then incorporated into the solution and thepH of the solution is adjusted to the range of about 1 to about 7 bymeans of ammonia. Iridium can be deposited using the insoluble anodeplating process upon a properly prepared metal substrate immersed in thebath using a cathode current density of about 0.1 to about 8 amperes persquare decimeter with a bath temperature in the range of about 50 toabout 75 C. Anodes such as platinum or platinized titanium may beemployed. Iridium deposits having a thickness up to about 10microinches, e.g., about 2 to about l0 microinches, can be produced fromthe bath which are adherent to the substrate and which are substantiallyfree from cracks. The bath prepared in the aforedescribed way is stableand can be stored for periods of months without encountering deleteriouseffects. Substrates which may be electroplated with iridium inaccordance with the invention include stainless steel, nickel basealloys, mild steel, copper, brass, gold, ironnickel-cobalt alloys, etc.in plating a substrate such as stainless steel, the steel surface shouldbe carefully cleaned and provided with a thin coating of nickel, forexample, from an acid nickel chloride plating bath containing about 240grams per liter of nickel chloride hexahydrate and about 36 grams perliter of HCl. The work treated in the bath is first made anodic forabout l k to 2 minutes and is then made cathodic for about 6 minutes tocoat the surface of the work with a thin nickel electrodeposit. Acurrent density of about to about a.s.f. is employed.

In order to give those skilled in the art a better understanding of theinvention the following example was given:

iridium trichloride containing about 54.13 percent by weight of iridiumwas dissolved in 2700 cc. of distilled water in the amount of 74.1 gramsof iridium trichloride. 296.3 grams of sulfamic acid were thenintroduced. The solution was refluxed at about 100 C. for about 30hours. The solution was then cooled to room temperature and 33.44 gramsof ammonium sulfamate were added. The pH of the solution was adjusted topH 6 using 223 cc. of concentrated ammonium hydroxide. The solutionvolume was then adjusted to 9 liters. A 40-mesh stainless steel screencathode having an area of about 18.9 square centimeters was prepared forplating by degreasing in acetone, cathodic alloy cleaning in an alkalinebath, followed by an anodic cleaning in an alkaline bath. The cleanedscreen was then immersed in an acid nickel strike bath containing about240 grams per liter of nickel chloride hexahydrate and about 36 gramsper liter of hydrochloric acid and was made anodic in the bath at ananodic current density of 25 amperes per square foot for L5 minutes.Without removing the stainless steel screen from the acid nickell strikebath it was then made cathodic in the bath for 6 minutes at a cathodedensity of about 25 amperes per square foot. A thin nickel coat was thusproduced on the stainless steel screen. The nickel-coated stainlesssteel screen was then electroplated with about I micron of ruthemium ina bath containing 12 grams per liter of ruthenium, l0 grams per liter ofammonium sulfamate, having a pH of about 1.5, a temperature of 70 C. andusing a cathode current density of IO milliamperes per squarecentimeter. The ruthenium coated screen was then immersed in a portionof the aforementioned iridium bath and was plated for 20 minutes at acurrent density of 3 milliamperes per square centimeter at a bathtemperature of 70 C. The resulting iridium deposit was about 1.5microinches in thickness. The deposit was essentially crack-free, wasstrongly adherent to the substrateand was metallic in appearance. Theplated stainless steel screen exhibited good catalytic activity in anexperimental antipollution device designed to reduce nitrogen oxides inautomobile exhaust gases.

In preparing the iridium solution by refluxing iridium trichloride withsulfamic acid, the solution should contain at least about 9 grams perliter and up to about 55 grams per liter of iridium and sulfamic acidshould be present in the solution in a molar ratio of sulfamic acid toiridium of about 7 to about 44. Refluxing should be conducted atsubstantially the boiling point of the solution for at least about l5hours and up to about 60 hours. Thirty hourshas been found to beadequate. Preferably the iridium content of the solution is about l3grams per liter, and the molar ratio of sulfamic acid to iridium isabout IS. The foregoing conditions must be adhered to as otherwiseiridium deposits of only poor adherence are obtained from the resultingbath.

A preferred bath in accordance with the invention contains about 12grams per liter of iridium, about 10 grams per liter of ammoniumsulfamate, is operated at a temperature of about 70 C. over a pH rangeof about 2.5 to about 6 at a cathode current density of about 0.3amperes per square decimeter. Normally, no agitation is employedalthough moderate agitation may be used without harmful effects.

It is to be understood that when a ratio is expressed herein, forexample, as being about 7 to about 44, ratios of about 7:l to about 44:l are expressed.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

lclaim:

l. The method for preparing an iridium electroplating bath whichcomprises digesting an aqueous solution of iridium trichloridecontaining about 9 to about 55 grams per liter of iridium with sulfamicacid in a molar ratio of sulfamic acid to iridium of about 7 to about 44for at least about l5 hours at a temperature of about C., cooling theresulting solution to about room temperature, adjusting the iridiumcontent of the solution to about 3 to about 20 grams per liter,introducing about 3 to about 20 grams of ammonium sulfamate into thesolution and adjusting the solution pH to about 1 to about 7 by means ofammonia.

2. The method according to claim 1 wherein the final bath contains aboutl2 grams per liter iridium, about 10 grams per liter ammonium sulfamateand has a pH of about 2.5 to about t of about 0.1 to about 8 amperes persquare decimeter to electrodeposit iridium at said cathode.

4. The method according to claim 3 wherein the bath contains about 12grams per liter of iridium, about 10 grams per liter of ammoniumsulfamate, has a pH of about 2.5 to 6 and a temperature of about C. andcurrent is passed at a cathode current density of about 0.3 amperes persquare decimeter.

2. The method according to claim 1 wherein the final bath contains about12 grams per liter iridium, about 10 grams per liter ammonium sulfamateand has a pH of about 2.5 to about
 6. 3. The method for electroplatingiridium which comprises immersing at least one insoluble anode and acathode to be plated in a bath prepared by digesting an aqueous solutionof iridium trichloride containing about 9 to about 55 grams per liter ofiridium with sulfamic acid in a molar ratio of sulfamic acid to iridiumof about 7 to about 44 for at least about 15 hours at a temperature ofabout 100* C., cooling the resulting solution to about room temperature,adjusting the iridium content of the solution to about 3 to about 20grams per liter, introducing about 3 to about 20 grams per liter ofammonium sulfamate into the resulting solution, adjusting the solutionpH to about 1 to about 7 by means of ammonia to form an iridiumelectroplating bath, adjusting the bath temperature to the range ofabout 50* C. to about 75* C., and passing current from said anode tosaid cathode at a cathode current density of about 0.1 to about 8amperes per square decimeter to electrodeposit iridium at said cathode.4. The method according to claim 3 wherein the bath contains about 12grams per liter of iridium, about 10 grams per liter of ammoniumsulfamate, has a pH of about 2.5 to 6 and a temperature of about 70* C.and current is passed at a cathode current density of about 0.3 amperesper square decimeter.